Attenuation equalizer



May ,1 I w. R. LUNDRY 2,374,872

ATTENUATION EQUALI ZER Filed April 7, 1943 2 Sheets-Sheet l W. R. LUNDR)BY y .1, 1945 w. R. LUNDRY 2,374,872

ATTENUATION EQUALI ZER Filed April 7, 1945 .2 Sheets-Sheet 2 PW f 244m;

A T TORNEV in a transmission either direction from over-all fiat lossmay Patented May 1, 1945 UNI ED STAT S PATENT ornca ATTENUATIONEqUAL'rzEa Walter R. Lundry, Maplewood, N. J., asslgnor to BellTelephone Laboratories, Incorporated, New York, N. Y., a corporation ofNew York Application April 7, 1943, SerlalNo. 482,081

28 Claims.

This invention relates to wave transmission networks and moreparticularly to variable attenuation equalizers for use as regulatingnetworks.

The principal object of the invention is to compensate for changes inthe attenuation distortion line or the like caused, for ex'- ample, byvariations in temperature or humidity.

'A feature of the invention is anattenuation equalizing network whichoperates over two separated frequency ranges and is independentlyvariable over one or ,both of the ranges.

The attenuation distortion in a telephone circuit varies continually dueto changes in temperature or humidity. Continuously variable attenuatlonequalizers which may be varied in a constant fiat loss are employed tocompensate for these variations. When such a circuit requiresequalization over two separated frequency ranges the practice heretoforehas been to use two equalizers, with a resistance pad lbetween, thusmore than doubling the flat loss of a single equalizer. By employingspecial coupling arrangements between the equalizers the be reducedsomewhat, but in long circuits the total becomes very large.

The attenuation equalizer in accordance with the present inventionoperates over two'separated frequency ranges but of a single equalizer.By proper design the equalizer may be made independently varialble overone or both of the ranges. The variable elements may, for example, beonly two variable resistors which are readily adapted for automaticregulation. I

The equalizer, which is of the general type disclosed in'United StatesPatent 2,096,027, issued October 19. 1937, to H. W. Bode, comprises avariable impedance coupling branch connected in series or in parallelwith the wave source impedance and the load impedance. The couplingbranch comprises a fixed resistor connected atone end of a subsidiaryfour-terminal bridged-T network terminated at its other end in avariable impedance which may be either a variable resistor or afour-terminal constant resistance buildingcut network terminated in avariable resistor. The subsidiary network comprises a reactive bridgingimpedance branch and a reactive shunt impedance branch which areinversely related and two series branch'es, either a variable resistor,a fixed frequency-dependent impedance or a variable frequency-dependentimpedance. This last-mentioned impedance may include a secondfour-terminal! introduces only the flat loss one of which may be Illconstant resistance building-out network terminated by a variableresistor. When two variable controls are provided the equalizer may bedesigned to provide substantially independent,

regulation over two sufllciently widely separated frequency ranges. v

The nature of the invention will be more fully understood from thefollowing detailed description and by reference to the accompanyingdrawings in which like reference characters refer to similar orcorresponding parts and in which:

Fig. 1 is a schematic circuit showing one form of the two-rangeattenuation equalizer of the invention, employing a series couplingbranch which comprises two building-out networks terminated in variableresistors:

Fig. 2 is a schematic circuit showing another form of the equalizersimilar to one shown in Fig. 1 except that the coupling branch isconnected in parallel;

Fig. 3 shows an equalizer circuit similar to the one of Fig. 1 exceptthat the building-out networks are omitted;

Fig. 4 is a circuit similar 'to the one shown in Fig. 2 but with thebuilding-out networks omitted:

Fig. 5 shows a circuit similar to the one of Fig.3 except that one ofthe variable resistors 'is replaced by a frequency-dependent impedance;v

Fig. 6 is a circuit similar to the one shown in Fig. 4 'but with afrequency-dependent impedance substituted for one of the variableresistors;

Fig. 7 is a schematic circuit showing the structure of Figs. 1 and 2formed into a bridged-T network; 1

Fig. 8 shows the structures of Figs. 5 and '6 formed into a bridged-lnetwork; and

Fig. 9 shows two equalizers of the type shown in Fig. l coupled byresistors to provide a fourrange network.

Taking up the figures in more detail, Fig. 1 shows a series type oftwo-range attenuation equalizer in accordance with the invention. Thenetwork, which is of'the same general configuration as the one shown inFig. 11 of the abovementioned Bode patent, has a pair or input terminalsl, 2 to which is connected a wave source of impedance Zs and voltage Eand a pair of output terminals 3, 4 to which the load impedance Zn isconnected. Connected in series with the impedances Z5 and Z1. is avariable impedance coupling branch 5 which comprises a fixed resistor ofvalue R and a four-terminal subsidiary network 6 .of the bridged-T typehaving two pairs of terminals 1. 6 and 9, ill. The resistor R isconnected. between the terminals I and I of the load impedances and theterminals I and 6 of the network 6 are connected respectively, to theterminals of the resistor R. The network 6 is terminated at its otherend in a variable impedance which comprises a four-terminal building-outnetwork N1 terminated in a variable resistor R1. The subsidiary network6 comprises a reactive bridging impedance Z1 and a reactive shuntimp'edance Z2, which are inversely related to each other with respect toR0, and two series branches. One of the series branches is the fixedresistor of value R and the other is constituted by a secondfour-terminal building-out network N: terminated in a secondvaria-bleresistor R2. The building-out networksNi and N: have image impedancesequal to the subsidiary network pedance of Ra.

The insertionfactor $00 for the network of Fig. 1 operating between theterminal impedances Zs and Zn, as shown, with the variable resistors R1and R: set so that i also has an image imi= 2 0 may be found from theexpression R 0 1+ H- (zs+z.. I (2) If no is the real part of 00 thediflerence between an and the insertion loss a, for any other settingsof the resistors R1 and R1, may be found to a suflicient degree ofaccuracy for design purposes from the approximate expression in which Reindicates that only the real part of the complex quantity in thebrackets is to be use, (p3 and pi are the transfer constants of thenetworks N1 and N2 respectively,

R0, andwhenRa is equal to RA! An inspection of Equation 3 shows/that,in-g'eneral, the presence of the building-out networks N1 and N2 reducesthe maximum loss swing that would be obtained if they were omitted. Itfollows, therefore, that the impedancs Z1 and Z2. which are general incharacter, must be chosen to give a somewhat greater swing than isdesired from the network as a whole. Furthermore, in order to insuresubstantial independence of regulation over two frequency ranges, Z1must be large compared to R0 over the range in which p: is to controlthe regulation and Z2 must be large compared to R0 over the range where91 is to control.

It is usually desirable that the change in insertion los a-ao besubstantially proportional to min the one frequency range and to p: inthe other range. From Equation 3 it is apparent that, in the range wherep1 controls, the desired result maybe accomplished in one of thefollowing ways: flrst,-'by setting R2 equal to Ray sec- 0nd, by makingthe real part of 2+o4) large compared to unity; or, third, by making thephase angle of pz+ i) equal to 45 degrees. Of course,

any two or all three of these conditions may be approximated at the sametime. On the other hand, in the range where p: controls either R1 is setat the value R41, the real part of (pH-(pa) is made large compared tounity or the phase angle of (oi-i-r a) is made equal to 45 degrees.

The two-range attenuation equalizer shown schematically in Fig. 2 is ofthe shunt type, having the same general configuration as shown in Fi s.15 and 2'7 of the above-mentioned Bode patent. The variable impedancecoupling branch H, which is connected between the points l2 and It inparallel with the load impedances Z5 and Zn, comprises a fixed resistorR: and a subsidiary bridged-T network 6 similar to the subsidiarynetwork used in the equalizer of Fig. 1. One terminal of the resistor R3is connected to the point i2 and the other terminal is connected toterminal I of the network 6, terminal 6 of which is connected to thepoint It.

The equalizer of Fig. 2 may be designed in accordance with theprinciples set forth above to give the sametype of regulation under thecontrol of the variable resistors R1 and-R: as is obtainable with theequalizer of Fig. l. The insertion factor, for the condition that R1 andR: are both set equal to R0, is given by the expression Ra ows-ln) s n(R1+ Rn) (8) The regulation characteristic for other settings of R1 andB: may be found from Equation 3 by multiplying either side by (l).

In some cases the building-out networks N1 and N: are not needed tioncharacteristic. Fig. 3, for example, shows the series type circuit ofFig. 1 modified by the omission of N1 and N2. The characteristic may befound from Equation 3 by setting v =0 (9) Fig. 4 shows the alternativeshunt typ circuit of Fig. 2 with the networks N1 and N2 omitted.

A useful modification of the circuits of Figs. 3 and 4 is to replace oneof the variable resistors by either a'fixed or a variablefrequency-dependent impedance. In Fig. 5 and Fig. 6, for example, thevariable resistor R2 is replaced by the frequency-dependent impedanceZ3, which may be either variable, as indicated by the arrow, or fixed.If 23 is fixed, the equalizer is, of course, variable over only onerange.

A two-range equalizer having at each end an image impedance R03 which isa constant resistance may be provided by combining series and shunt typenetworks in a bridged-T structure. Fig. '7 shows such a bridged-T, withinput terminals l4, l5 and output terminals l6, l1, comprising two equalseries resistors Ra. Ra an in terposed shunt branch l8 connected betweenthe points I9, 20 and a bridging branch 2i connected to the terminals22, 23. The bridging branch. 2 I, which is similar to the seriescoupling branch 5 of Fig. 1, comprises a subsidiary bridged-T network 24of constant resistance image impedance R01, with terminals 22, 28 and25, 26, similar to the network 6 of Fig. l. The series arms of thenetwork 24 are constituted by a fixed resistor of value R01 and abuilding-out network N12, also of constant resistance image impedanceR01. terminated in a variable resistor R 2. The bridgin impedance Z11and theshunt impedance Z1: have the relationship.

V ZuZis=R01 (10) The network 24 is terminated at its terminals 26.

to give the desired regulaasvas'm 3 It in a second building-out networkNu. of constant resistance image impedance Roi which, in j turn, isterminated in a variable resistor R11. In the bridging branch II theresistor which corresponds to R in Fig. 1 has been incorporated in the 5T of resistances constituted by Rs, R. and Rh, in order to save oneelement.

The shunt branch it, which is similar to shunt ance image impedance Ros,with terminals 2|.

29 and 30, ll similar to the network I of Fig. 2.

The series arms of the network 21 are constituted by a fixed resistor ofvalue R02 and athird. building-out network N22, also of constantresistance image impedance Roz, terminated in a variable resistor R221.The bridging impedance Zn and the shunt impedance Z22 have therelationship i4, i5 and i6, I I of the equalizer of Fig. 7, the

image impedances R01 and R0: of the auxiliary and the building-outnetworks must have the relationship Ro1Roa=Rca= (12 and for all settingsthe values of the variable resistors must have the relationship In orderto facilitate the maintenance ofthis relationship conveniently theresistors Ru and R21 may be placed under a unitary control, as,

indicated by the broken line 32, and the resistors R12 and R22 under asecond unitary control, as

indicated by the broken line 33.

The series type equalizer of Fig. 5. and the shunt type equalizer ofFig. 6 may also be combined in a constant resistance bridged-Tstructure, as shown in Fig. 8. The circuit of Fig. 8 is similar to theone shown in Fig. '7 except that the building-out networks Nu, Nu, N21and Na are omitted and the variable resistors R1: and

R22 replaced respectively, by the general impedances Z1: and Zn whichcorrespond to the impedance Z: of Figs. 5 and 6 respectively. and havethe relationship Z13Z2a=Ros (14) In order to provide an adjustment overthe second frequency range the impedances Zn and Zn may be madeadjustable as indicated by the arrows. If the constant resistance imageim- P dance R03 is to be maintained the values of the impedances Z13 andZ2: must satisfy Equation 14 for all settings and, therefore, theseimpedances may conveniently be arranged for unitary control, asindicated. The impedance Z1: and Z2: may, of course, be simple variableresistors such as R: in Figs. 3 and 4.

Fig. 9 shows two networks at and II of the type shown in Fig. 1connected in tandem between the terminal loads Z5 and Zn to provide anequalizer which may be designed to give independent regulation over fourfrequency ranges. I'hese ranges may, if desired, coincide in pairs togive in eflect double regulation over two ranges. the resistor R and thebridged-T auxiliary net- The resistors R5 and Re correspond to.

works II and I! to the network I, of Fig. 1. The two equalizers 84 andl! are coupled by means of a r type resistance pad I. made up of the twoshunt resistors R1, Re and the interposed series resistor Rn. It will benoted that the leads 3.

' and 40 from the networks It and II respectively.

cross each other as they go to the terminals ll and 42 respectively, ofthe resistor R0. With this configuration and proper design the over-allflat loss may be reduced from the value it would have if the leads I!and 40 were connected respectively, to the terminals 42 and II. Thistype of coupling is more fully described in the copending United Statespatent application, Serial No. 461,- 171,- filed October 7, 1942.

What is claimed is:

l. A variableattenuatlon equalizer comprising in combination with a wavesourceimpedance and a load impedance a coupling branch interposedbetween said. impedances, said coupling branch comprising a subsidiaryfour-terminal bridged-T networ a resistor connected at one end of saidsubsidiary network and a variable impedance terminating said network atits other end, said network comprising two series branches. aninterposed reactive shunt impedance and a reactive bridging impedance,said shunt impedance and said bridging impedance being inversely relatedto each other and one of said series branches having afrequency-dependent impedance characteristic.

2. An equalizer in accordance with claim 1 in which said interposedcoupling branch is connected in series with said wave sourceimpedanceand said load impedance.

3. An equalizer in accordance with claim 1 in which said interposedcoupling branch is connected in parallel with said wave source impedanceand said ioadimpedance.

4. An equalizer in accordance with claim 1 in which said resistor isconnected in series with said wave source impedance and said loadimpedance and a pair of terminals of said'subsidiary network areconnected respectively to the terminals of said resistor.

5. An equalizer in accordance with claim 1 in which one terminal of saidresistor is connected to one terminal of said wave source impedance andone terminal of said load impedance, the other terminal of said resistoris connected to one terminal of said subsidiary network and anotherterminal oi said subsidiary network is connected to the other terminalof said wave source impedance and the other terminal of said loadimpedance. v

6. An equalizer in accordance with claim 1 in which said one seriesbranch is variable in impedance.

7. An equalizer in accordance with claim 1 in which said one seriesbranch comprises a fourterminal building-out network.

8. An equalizer in accordance with claim 1 in whichsaid one seriesbranch comprises a fourterminal building-out network terminated by avariable impedance.

9. An equalizer in accordance with claim 1 in which said one seriesbranch comprises a fourterminal building-out network of constantresistance image impedance terminated by a variable resistor.

10. An equalizer in accordance with claim 1 in which said variableimpedance comprises a fourterminal building-out network.

11. An equaliz'er'in accordance with claim 1 in Y which said variableimpedance comprises a fourterminal building-out network of constant re-1 which said one series branch and impedance each comprises atour-terminal buildterminal building-out .ing-out network ingeoutnetwork resistance image sistance image impedance terminated by avariable resistor.

12. An equalizer in accordance with claim 1 in which said one seriesbranch and said variable impedance each comprises a tour-terminalbuildmg-out network.

13. An-equalizer in accordance with claim 1 in said variable ing-outnetwork of constant resistance image impedance terminated by a variableresistor.

1 14. An equalizer in accordance with claim 1 in which the other or saidseries branches is a resister of value Rio-and said shunt and bridgingimpedances are inversely related with respect to-Ro.

15. An equalizer in accordance with claim 1 in which said variableimpedance comprises a fournetwork of constant re-' sistance imageimpedance Ru terminated by a variable resistor and the other of saidseries branches is a resistor of value R0.

l6. An equalizer" in accordance with claim 1 in which said one seriesbranch comprises a fourterminal'building-out network of constantresistance image impedance R terminated by a variable resistor and theother of said series branches is a resistor of value Ro.

' 1'1. An equalizer in accordance with claim 1 in which said oneseriesbranch and said variable impedance each comprises a tour-terminalbuildof constant resistance image terminated by a variable resistor ofsaid series branches is a resistor impedance R0 and the other of valueRo. I

18. An equalizer in accordance with claim 1 in which said one. seriesbranch and said variable Rn over one trequency range and said shunt im-'pedance has a value which is large compared to R0 over a secondfrequency range.

20. A bridged-T attenuation equalizer of constant resistance imageimpedance Ros comprising a bridging branch and a-shunt branch, each ofsaid branches comprising a subsidiary tour-terminal bridged-T networkand a variable impedance terminating said network, each or said networkscomprising two series branches, an interposed reactive shunt impedanceand a reactive bridging impedance,'said shunt impedance and saidbridging impedance being inversely related to each other and one of saidseries branches having a frequency-dependent impedance characteristic.

21. An equalizer in accordance with claim in which said subsidiarynetworks have image impedances inversely related with respect to R03.

7 22. An equalizer in accordance with claim 20 in which said one seriesbranch is variable in impedance.

impedance each comprises a four-terminal buildof constant resistanceimage impedance Ru terminatedby a variable resistor, the other of saidseries branches is a resistor of value R0 and said shunt and bridgingimpedances are inversely related with respect to Re. I

- 19. An equalizer in accordance with claim 1 inv which said one seriesbranch is variable in impedance, said subsidiary network has a constantimpedance Rn when said one series branch. is set equal to R0, saidbridging impedance has a value which is large compared to 23. Anequalizer in accordance with claim 20 in which said one series branchcomprises a fourterminal building-out network. v

24. An equalizer, in accordance with claim 20 in which said one seriesbranch comprises a tourterminal building-out network of constantresistance image impedance terminated by a variable resistor.

'25. An equalizer in accordance with claim 20 I in which said variableimpedance comprises a tour-terminal building-out network.

26. An equalizer in accordance with claim 20 in which said variableimpedance comprises a four-terminal building-out network of constantresistance image impedance terminated by a variable resistor.

2'1. An equalizer in accordance with claim 20 in whichsaid one seriesbranch and said variable impedance each comprises a four-terminalbuilding-out network.

28. An equalizer in accordance with claim 20 in which said one seriesbranch and said variable impedance each comprises a four-terminalbuilding-out network of constant resistance image impedance terminatedby a variable resistor.

warrraa R. LUNDRY.

